The rapid technology advancements in high performance electronics packaging have focused on reduced size and higher operating speed. This has resulted in excessive heat generation during device operation. There is an accompanying need for effective heat dissipation methods to maintain the reliable functional performance of assembled electronic products. The commonly used methods of cooling include helium filled modules, solder thermal interfaces, thermal greases, elastomeric silicone gels, thermoplastic polymers with thermally conductive fillers such as AlN, BN, ZnO, and more recently, phase change materials (PCM), and conductive adhesives. These provide the thermal interface between the silicon device chip and a high thermal conductivity metal heat spreader or heat sink to allow a path for heat dissipation from the high power density circuit devices during operation.
Thermal grease is spread as a thin layer between the back of the die and the heat sink. Thermal grease has low thermal resistance and can be easily reworked. However, it is subject to pump-out and drying, which causes voids at the interface. This degrades the device performance with time due to an increase in interfacial resistance. Phase change materials (PCM) are low melting waxes. Examples include paraffin wax having graphite particles dispersed in the wax matrix, and silicone based materials, such as alkyl methyl silicones, which can be used as pre-formed tapes or melt dispensed across interfaces. PCM's provide low thermal impedance and high thermal conductivity, typically in the range of about 3-5 W/m ° K in thin bond line thickness. However, the pre-cut films of these materials are fragile and also have the problems of performance degradation and variability, delamination, bleed-out, and out-gassing, and furthermore generally require fasteners, such as clips or screws to hold the PCM in place.
Another category of thermal interface materials is conductive adhesives, which can be used as a thin adhesive interlayer between the heat sink or the heat spreader and the backside of a silicon die in a flip-chip module assembly. The commercially available conductive adhesives are typically Ag-filled and ceramic-filled epoxy-based materials including flexible epoxies. The epoxy-based materials are medium to high modulus adhesives (>100,000 psi at room temperature). It is generally known that cured coatings of such materials have high intrinsic stress, which can cause disruption of interface integrity due to delamination. This results in increased contact resistance with a corresponding decrease in the heat dissipation effectiveness at the interface. The commercially available Ag-filled epoxy adhesives also have no simple and practical rework method available. Therefore, the Ag-filled epoxy adhesives cannot be readily removed or reworked from contacting surfaces. The non-reworkability of these epoxy adhesives presents a serious drawback in that it does not allow for defect repair, component recovery, recycling or reuse of high cost semiconductor devices, heat sinks and substrates.
The ability to rework and recover components to recover production yield loss, reduce waste, and provide cost reduction has become more important in the fabrication of high performance electronic products. A rework option for a cured adhesive-type thermally conductive film offers the major benefit of recovery/reclamation and reuse of potentially expensive high thermal conductivity heat spreader materials, sensitive components, or voltage transformation modules. Moreover, a rework option may provide a cost effective way to obtain significant increases in heat dissipation capability with the use of high thermal conductivity cooling elements in conjunction with a thermal interface adhesive.
Accordingly, desirable properties for adhesive-type thermal interface materials would include: the ability to form a thin bond line with uniform thickness across interfaces, low thermal impedance, low stress and compliant systems for interface integrity during device operation, stable interfacial contact resistance in T/H (temperature/humidity) and T/C (thermal cycling), TCR (temperature coefficient of resistance) stability, and reworkability for defect repair and reclamation of high cost module components. Accordingly, the preferred adhesive-type materials should also be amenable to removal from contacting surfaces to allow reworking without causing any damage to the module materials, particularly special type heat spreaders having high thermal conductivity.
Moreover, thermally conductive adhesive films should also be chemically stable at the operating conditions and during rework. Under typical operating conditions, the cured thermally conductive films are subjected to heat cycling, which can induce thermal degradation of the components making up the cured thermally conductive films. One common degradation pathway is hydrolytic decomposition facilitated by adventitious atmospheric water, which is accelerated at elevated humidity and elevated temperature.
Accordingly, in view of the limitations in the use of conventional interface materials, there is a need for improved thermal interface materials (TIMs) with efficient heat dissipation from high power density devices, and that possess desirable hydrolytic stability and reworkability properties. There is also a need for a practical method to remove and/or rework the cured deposits/residue of these materials from various component surfaces/interfaces to which the materials are adhered.